[Paper Review] Nanoindentation of single crystalline Mo: Atomistic defect nucleation and thermomechanical stability

F. J. Domínguez-Gutiérrez, Stefanos Papanikolaou+3 more|arXiv (Cornell University)|May 17, 2021

Metal and Thin Film Mechanics53 references40 citations

TL;DR

This study uses molecular dynamics simulations to investigate the thermomechanical stability of single-crystalline molybdenum under spherical nanoindentation at temperatures from 10 to 800 K. It identifies the formation and exceptional thermal stability of [001] dislocation junctions—particularly those formed by 1/2[111] dislocations—as the key atomic mechanism responsible for molybdenum's persistent high-temperature hardness, distinguishing it from other BCC metals like W and Ta.

ABSTRACT

The mechanical responses of single crystalline Body-Centered Cubic (BCC) metals, such as molybdenum (Mo), outperform other metals at high temperatures, so much so that they are considered as excellent candidates for applications under extreme conditions, such as the divertor of fusion reactors. The excellent thermomechanical stability of molybdenum at high temperatures (400-1000$^{ m o}$C) has also been detected through nanoindentation, pointing towards connections to emergent local dislocation mechanisms related to defect nucleation. In this work, we carry out a computational study of the effects of high temperature on the mechanical deformation properties of single crystalline Mo under nanoindentation. Molecular dynamics (MD) simulations of spherical nanoindentation are performed at two indenter tip diameters and crystalline sample orientations [100], [110], and [111], for the temperature range of 10-1000K. We investigate how the increase of temperature influences the nanoindentation process, modifying dislocation densities, mechanisms, atomic displacements and also, hardness, in agreement with reported experimental measurements. Our results suggest that the characteristic formation and high-temperature stability of [001] dislocation junctions in Mo during nanoindentation, in contrast to other BCC metals, may be the cause of the persistent thermomechanical stability of Mo.

Motivation & Objective

• To understand the atomic-scale origins of molybdenum’s exceptional thermomechanical stability at high temperatures (400–1000 °C), particularly in extreme environments like fusion reactor divertors.
• To investigate how temperature influences dislocation nucleation, evolution, and junction formation during nanoindentation in single-crystalline Mo.
• To clarify why Mo maintains high hardness at elevated temperatures while other BCC metals (e.g., W, Ta) do not, despite similar crystal structures.
• To link observed macroscopic mechanical behavior (e.g., hardness) to specific atomistic defect mechanisms, especially dislocation junctions.
• To provide a mechanistic explanation for Mo’s low degradation and high performance under extreme thermomechanical loading, using atomistic simulations validated against experimental trends.

Proposed method

• Performs large-scale molecular dynamics (MD) simulations of spherical nanoindentation on single-crystalline Mo with [100], [110], and [111] crystal orientations.
• Simulates temperature effects across 10–800 K using a repulsive imaginary indenter to avoid atomic relaxation issues and enable controlled loading.
• Tracks dislocation density, Burgers vector evolution, and atomic displacements as functions of temperature and indenter displacement.
• Analyzes the formation and stability of dislocation junctions, particularly [001] junctions formed from 1/2[111] dislocations, using topological and energetic criteria.
• Correlates simulated hardness values with dislocation density and junction formation to validate against experimental nanoindentation data.
• Uses supplementary visualization and time-resolved analysis to track dislocation nucleation and junction proliferation during loading and unloading.

Experimental results

Research questions

• RQ1 How does increasing temperature affect dislocation nucleation, propagation, and junction formation in single-crystalline molybdenum during nanoindentation?
• RQ2 Why does molybdenum maintain high hardness at elevated temperatures while other BCC metals (e.g., W, Ta) do not?
• RQ3 What is the role of [001] dislocation junctions in enhancing the thermomechanical stability of Mo under nanoindentation?
• RQ4 How do different crystallographic orientations ([100], [110], [111]) influence the temperature-dependent mechanical response and dislocation mechanisms in Mo?
• RQ5 To what extent are the simulated hardness values and dislocation dynamics consistent with experimental nanoindentation measurements at high temperatures?

Key findings

• The formation of thermally stable [001] dislocation junctions—specifically those formed by the reaction of two 1/2[111] dislocations—is identified as the dominant hardening mechanism in Mo at high temperatures.
• These [001] junctions are highly stable and proliferate under load, especially at temperatures above 400 K, correlating directly with increased simulated hardness.
• The number of [001] junctions increases significantly with temperature and indenter displacement, peaking at maximum load, indicating a temperature-activated strengthening process.
• In contrast to W and Ta, where such junctions are unstable or absent, Mo exhibits a unique combination of kinetic and energetic favorability that stabilizes [001] junctions.
• The simulated hardness values show good agreement with experimental measurements, validating the model’s predictive capability for high-temperature mechanical behavior.
• The [001] junctions are energetically favorable and thermodynamically stable once formed, explaining their persistence and contribution to strain hardening in Mo.